[Fixed maybe?] YouTube hasn't worked in Firefox for me for weeks

EDIT: It's something to do with my VPN and/or EasyTether, but it only happens in Firefox, so I didn't think that could be it. I commented about my findings below: https://lemmy.world/comment/7585497

I start a video and it either doesn't load at all of stalls within 5 seconds, never to recover.

I've tried everything:

• Deleting every Mozilla folder in AppData to completely refresh Firefox. (after backing up my profile)
• Flushing DNS cache
• Uninstalling and reinstalling Firefox. (Which made me remember I had a policies.json file in the program folder to permanently lockdown the settings I want. So that wasn't the problem either.)

Even when using no extensions like an ad blockers it still won't play videos. Besides, I'm a Premium subscriber. Once in a while a video will work, but it's so, so rare. And often it'll even stop after a minute.

Meanwhile, it'll work in the DuckDuckGo Browser all the time, which is Chromium based.

What do you think the deal is? It broke rather suddenly a few weeks ago. Do you think it's Mozilla's fault, or Google's?